1. Field of the Invention
The present invention concerns the adaptation of internal combustion engines to driving a propeller and relates more particularly to a belt-drive speed reducer, lodged in a housing, permitting reconciliation of motor speed to propeller speed.
2. Description of the Prior Art
Standard automobile engines are more efficient at speeds between 4000 and 6000 RPM. To apply these engines to the driving of an airplane propeller, it is necessary to reduce the speed in a ratio ranging from 2.5 to 1 to improve the efficiency of the propeller and, at the same time, to reduce the propeller noise.
Many types of reducers have already been realized. Among them can be particularly cited spur-gear reducers. However, because they are very heavy, their use has gradually declined. Chain-drive reducers are reputed to be unreliable, so that their use on aircraft is actually believed to be illegal. There have also been conceived continuous speed changers, based on the rolling of smooth cylinders, cones or balls in tight contact. But these polished bodies, with point contacts, either flatten under the pressure necessary for transmission of the force or slip with respect to one another, this in spite of the use of special lubricants the viscosity of which increases with the pressure, the traction obtained not being sufficient for effective operation of the reducer.
A first category of reducers, driven by belts, presents itself more favorably. These reducers consist very simply of a drive pulley turned by the motor crankshaft, this drive pulley transmitting its rotary motion, through the intermediary of a belt, to a driven pulley, itself turning a propeller shaft. The assembly is enclosed in a protective housing attached to the motor.
The belts commonly used are certainly quite durable, but they overheat, due to slippage, particularly in the resonance regime of the motor. The use of a tensioning idler pulley is not to be considered for several reasons, one being that installing it inside the housing presents difficulties and the two supplementary flexions, which it imposes on the belt, would result in premature fatiguing of the latter while decreasing the contact arcs around the drive and driven pulleys, thus favoring slippage. Besides, in view of the linear speed of the belt, a small diameter tensioning idler would turn at a very high angular velocity, poducing excessive parasitic noise. Finally, in a belt reducer driven from an automobile engine crankshaft, the latter does not bear well the lateral tension exerted on it by the belt without important technologic modifications.